Histamine production during the anti-allograft response. Demonstration of a new lymphokine enhancing histamine synthesis.

Histamine production is greatly increased during culture of allograft recipient spleen cells in the presence of immunizing cells (secondary mixed leukocyte cultures [MLC]) as compared to that found in primary MLC (i.e., without previous allograft). This phenomenon appears after 24 h of culture and reaches its maximum at 48 h. Optimal increased histamine production is observed when MLC is performed with spleen cells removed from mice during rejection. This increased production of histamine during secondary MLC results from the action of a lymphokine: the histamine-producing cell stimulating factor (HCSF). This factor is released by T lymphocytes. Its production requires specific stimulation of the recipient lymphocytes because increase in histamine production during secondary MLC can be only observed when recipient cells are cultured with stimulating cells bearing at least one homology at K or D loci with immunizing cells. HCSF acts on a cell which is present in bone marrow, spleen, blood, and peritoneal cells but absent in thymus or lymph node cells. This target cell is found in the less-dense layer of a discontinuous Ficoll-gradient of bone marrow cells. HCSF is heat stable, destroyed by trypsin treatment, and has a molecular weight between 50,000 and 100,000. It acts on its target cells by increasing histidine decarboxylase activity.